Electrons hold the world together. When chemical reactions yield new substances, they play

a leading role. And in electronics, too, they are the protagonists. Together with his colleagues,

Ferenc Krausz, Director of the Max Planck Institute of Quantum Optics in Garching,

photographs the rapid movements of electrons with attosecond flashes, creating the basis

for new technological developments.

The Making of a Quantum Movie

TEXT ROLAND WENGENMAYR

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T he black curtain goes up: As if on a stage, below us lies a clean-room, about the size of a school gymnasium. It is almost entire-ly filled with a laser system.

Here, powerful laser beams zip through the air, containing a stream of femtosec-ond light pulses lasting just a few mil-lionths of a billionth of a second. The sys-tem is so sensitive that we can only ad-mire it through an observation window. The complex assembly of optical instru-ments before us forms the first section of a speedway where the world’s shortest flashes of light cross the finish line. But this is no racetrack: we’re in the depart-ment of Ferenc Krausz at the Max Planck Institute of Quantum Optics in Garching.

The femtosecond laser pulses – which are already very short – travel through a vacuum tube to a lab one floor down. We’re allowed to enter this lab. This is where attosecond flashes are created, which are a thousand times shorter than femtopulses. In the lab stands a large barrel to which doctoral students Martin Schäffer and Johann Riemens-berger are making some adjustments. The barrel is vaguely reminiscent of an enormous washing machine drum crossed with an old diving helmet. In reality, it’s a vacuum chamber made of solid stainless steel. Its armored-glass windows permit a glimpse of the probe that marks the target of the attosecond flashes of light.

Schäffer and Riemensberger belong to the Garching-based team of Reinhard Kienberger, who is also a physics profes-sor at Technische Universität München (TUM). More than a hundred scientists conduct their research in Ferenc Krausz’s department. The pioneer of attosecond technology attracts young scientists from all over the world who want to observe some of nature’s extremely fast processes.

The long path for attopulses: Manish Garg (left) and Antoine Moulet work at the experiment bench over which the attosecond pulses are directed. Because the photo was taken with an extremely wide-angle lens, the straight bench appears curved.

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selves to this topic. Eadweard Muy-bridge solved the mystery: yes, for a brief moment, all four hooves are in the air. In 1878, Muybridge succeeded for the first time in freezing all phases of a horse’s gallop in rapid snapshots.

Since then, high-speed images of processes that happen faster than the eye can see have expanded our knowl-edge considerably. In natural sciences, the decryption of ultrafast processes in the world of atoms and elementary par-ticles is even essential.

ELECTRONS BEHAVE LIKE SHELL-GAME TRICKSTERS

Attosecond research in Garching focus-es on electrons. “These are practically everywhere,” comments Krausz on the predominance of this negatively charged elementary particle. Although electrons are extremely small, even from the perspective of the nano-world, they are the quantum glue that joins atoms together to construct the diverse forms of matter that make up our world. Moreover, electrons play a leading role in chemical reactions. New

substances are always created by elec-trons being pushed in between differ-ent atoms. Until recently, though, it wasn’t possible to trace the activity of these elementary particles directly due to the speed at which they act.

In other words, for researchers, electrons had long behaved like shell-game tricksters, fooling onlookers with their speed. Quantum mechanics can describe their behavior theoretically, but primarily just the initial and final situations in these electronic shell games. This was long the case for ex-periments, too: the electrons’ actual motion remained hidden. Not only was this gap in knowledge unsatisfac-tory from the point of view of basic re-search, but even today, it also impedes new, practical developments. Take cat-alysts, for example, which make chem-ical reaction processes more efficient. In order to systematically identify the best material to provide chemical assis-tance, researchers must be able to un-derstand just how a catalyst influences electron transfer.

As lasers became more and more powerful in the 1970s, a fascinating

Krausz is therefore the perfect person to convey a sense of how fast an attosec-ond is. Purely mathematically, an atto-second is a billionth of a billionth of a second. Even as a physicist, Krausz finds this numbers game lacking in clarity. So he searches for suitable com-parisons. “The fastest thing we know is light,” he explains, “which can circle the earth approximately eight times in one second.” But even though light is so fast, within one attosecond, it could only travel from one end of a single wa-ter molecule to the other!” With a di-ameter of only 0.3 nanometers (a nano-meter is one millionth of a millimeter), a water molecule is unimaginably tiny. “Nano” is derived from the Ancient Greek word for dwarf, nanos, and that is precisely what attosecond physics is about: extremely dwarfish quantum objects that move incredibly quickly.

Anyone who wants to understand the motivation of scientists like Ferenc Krausz in a historical context should consider the horse. For centuries, it was disputed whether all four of a horse’s hooves left the ground when galloping. Generations of painters devoted them- P

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Clean air for intense laser light: Tim Paasch-Colberg is working on the laser system in the cleanroom, where intense femtosecond pulses are produced. Any dust in the air could destroy the delicate optics.

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idea arose: perhaps extremely short la-ser pulses could be used to take snap-shots of chemical reactions, as Muy-bridge once photographed the phases of the horse gallop. “Individual images taken at different times can then be used to create a film that shows the pre-cise reaction process,” explains Krausz.

This approach created a new field of research based on what is known as the pump-probe method. In this meth-od, a first laser flash initiates a process, such as a chemical reaction, and after a precisely determined delay, a second flash takes a snapshot. By repeating the reaction and varying the delay between the pump and the probe laser pulses, the scientists obtain images of every stage of a process. They can then com-pile these into a film.

By the 1990s, this experimental technique was so sophisticated that it provided completely new insights into nature, making the movement of at-oms in molecules visible for the first time. For this pioneering achievement, Egyptian-American physical chemist Ahmed Zewail was awarded the Nobel Prize in Chemistry in 1999.

At the same time, however, what had until then been rapid development toward ever shorter shutter speeds ran up against a wall. Although the tech-nique was capable of showing process-es that occur in just a few femtosec-onds in slow motion, it was only the movements of atoms that could be filmed, not those of electrons, which are much lighter and faster. In order to freeze the latter in a snapshot, a radi-cal new attosecond flash technology was needed.

Around the turn of the millennium, Ferenc Krausz, then at Technische Uni-versität Wien, was the first to succeed in breaking the barrier to the attosec-

ond range. Since then, there have been steady improvements in filming the movement of electrons, and a new field of research has emerged. The sci-entists in Garching are now capturing the speedy world of electrons on film. But why did this technological barrier in ultrashort-time research exist?

FEMTOSECOND LASER LIGHT JOLTS ATOMS

“It has to do with the color of the light – in other words, its wavelength,” ex-plains Krausz. A light pulse can’t travel through space unless it comprises at least one wave crest and one wave trough. Only pulses from such a com-plete wave train are inherently so sta-ble that they can make their way from the laser to the probe without com-pletely dissipating. However, if a wave train is to be squeezed into an interval of a few attoseconds, its wavelength must be sufficiently short.

“For pulses of less than one femto-second, extreme ultraviolet laser light is needed,” says Krausz, “and to produce even shorter pulses, there’s no way around using X-ray light.” And that was precisely the problem: for a long time, there were no lasers that could produce light in this shortwave spectral range. Only in recent years have so-called free-electron lasers made advances into this range. But they have one drawback: they require kilometer-long particle accelera-tors – enormous, expensive facilities for large-scale research.

In the 1990s, Krausz – like Muy-bridge in his time – took on the chal-lenge of clearing this hurdle using exist-ing technology. Back then, Muybridge combined his giant plate cameras into one long row, each being triggered in succession by a horse galloping past.

Thus he managed to capture an image of each phase of the gallop.

Krausz had to make do with femto-second laser technology that had too long a wavelength. He hit upon the idea of shooting the high-intensity laser puls-es at noble gas atoms. The femtosecond laser light jolts the atoms and forces them to emit a flash in the ultraviolet or X-ray range. Because the atoms oscillate precisely in time with the excitation

Electrons passing through: Attosecond pulses from extremely ultraviolet light (violet) catapult electrons out of the atoms of a tungsten crystal (green). Using infrared laser pulses (red), the researchers in Garching measured that an electron takes 40 attoseconds to pass through a single layer of magnesium atoms (blue) above the tungsten crystal.

» Around the turn of the millennium, Ferenc Krausz was the first to succeed in breaking

through the barrier to the attosecond range. Since then, there have been steady improve-

ments in filming the movement of electrons, and a new field of research has emerged.

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femtosecond laser pulses, this shortwave light happens to have the pure, perfect quality of laser light (see MAXPLANCKRE-SEARCH 2004, vol. 2, p. 30). This trick al-lowed Krausz to advance into the atto-second range for the first time.

FASTEST FLASH IN GARCHING: 72 ATTOSECONDS

Since then, the scientists in Garching have continuously improved their atto-second facilities. In 2004, Krausz’s group achieved the world record for the shortest light pulse, at 650 attoseconds. Today, the best experiments flash around ten times faster. “With Martin Schultze’s team, we here in Garching have pared this down to 72 attosec-onds,” says Krausz, “but since 2012, Zenghu Chang’s group at the Universi-ty of Central Florida holds the world re-cord, with a mere 67 attoseconds.” The Max Planck Director isn’t bothered that

Garching lost the record – on the con-trary. “This shows that our research field is the subject of growing interest,” he explains proudly. In the end, it’s not about records, but about increasingly refined tools for studying the fastest movements in the microcosm.

Among the first subjects of attosec-ond metrology or attosecond chronos-copy, as the specialists in Garching call their technology, were the movements of electrons in individual atoms and molecules. However, for a few years now the researchers have been focus-ing on far more complex electron worlds: crystalline solids. In many in-organic materials, the atoms form the regular, spatial grid of a crystal. Crys-tals occur in great variety in nature; even the metals and semiconductors that are so useful for countless appli-cations form crystals.

With the new attosecond measuring technology, the team headed by Rein-

hard Kienberger has just recorded in real time how electrons zoom through a single layer of metal atoms. This pas-sage is of key importance for both phys-ics and technology. Its direct observa-tion can one day aid in developing substantially faster electronic switching elements and microprocessors.

Basic researchers are interested in the complex behavior of the electrons in crystals because the elementary par-ticles in them have two very central ef-fects that together determine many properties of matter. For instance, some of the electrons bind atoms to-gether to form a crystal lattice. They essentially make up the quantum glue of matter. The second effect is caused by electrons that can detach from their original atoms. In metals, for example, they whiz more or less freely through the crystal lattice, and can thus trans-port electric current. In addition, these conduction electrons have a major in-

HUNTING TUMORS WITH FEMTOSECOND TECHNOLOGY

Ferenc Krausz’s team is investigating, together with scien-tists from Ludwig Maximilian University of Munich (LMU) and Technische Universität München (TUM), how femtosec-ond lasers can help fight cancer. The researchers, from the fields of medicine, biology, informatics and physics, want to detect and treat cancer at an early stage, before the primary tumor can metastasize.

In the first step of the method, the Munich-based re-searchers intend to use special femtosecond lasers that pro-duce high-intensity infrared radiation. They can use this to de-tect the molecular building blocks of cancer cells and their metabolic products as an unmistakable fingerprint in blood and breath. Even in early stages of tumor development, indi-vidual cancer cells enter the bloodstream, and their metabol-ic products make their way into the breath through the lungs.

The next step is to scan patients using well-bundled X-rays that are gentle on tissue. These are produced by a high-in-tensity femtosecond laser. X-rays with such properties are currently available only in large accelerator facilities known as synchrotrons. Because they have very short wavelengths, they can make tumors visible that are smaller than one mil-limeter: ten times smaller than tumors that conventional X-ray machines can detect today. And that’s the crux: in a pri-mary tumor that small, there is a high probability that the cancer hasn’t yet spread. Consequently, destroying it actual-ly does mean healing the patients.

For this therapy, Krausz and his colleagues plan to irradi-ate tumors with electrically charged atoms, known as ions. These are also produced by the laser. This method is particu-larly efficient, yet gentle on healthy tissue.

» Among the first subjects of attosecond metrology were the movements of electrons in

atoms and molecules. For a few years now, researchers have been focusing on more complex

electron worlds: crystalline solids.

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fluence on the mechanical and optical properties of a material, and its ability to conduct heat.

In many crystals, however, there are no conduction electrons. These materi-als are thus known as insulators, one example of which is quartz. Semicon-ductors are a sort of hybrid between in-sulators and electrical conductors. As a material used in electronics construc-tion, semiconductors have radically changed our culture. In these materials, the electrons need a small push to be able to flow as conduction electrons. To understand this behavior, we must look at electrons as quantum particles.

Since an electron is also an electro-magnetic wave, it has a wavelength, which is linked to its kinetic energy – a bit like the sound of a racecar is linked to the speed of its engine. If an electron wants to move freely through a crystal lattice, its wavelength must match the spatial grid of the atoms. This is appli-cable for only a certain range of wave-lengths – and a certain range of energy. This range forms a kind of highway on

which electrons can race through the crystal. In metals, there is always a lot of electron traffic flowing in this con-duction band. In semiconductors, how-ever, even the most mobile electrons cling tightly to their atoms. They re-quire an energy kick to manage the quantum leap into the conduction band. This kick is used, for example, to switch operations in transistors.

DOES INTENSE LIGHT MAKE A QUARTZ CRYSTAL CONDUCTIVE?

In insulators, however, this energy kick would have to be so intense that it would rip the material. Could one nev-ertheless make an insulator conduc-tive, even if only for a short time? And would it also be interesting from a technological standpoint?

These are the questions a Garching-based team of attosecond researchers, in which Elisabeth Bothschafter and Martin Schultze were doing their doc-toral work, asked themselves. To find answers, the laser physicists needed

well-versed crystals experts on their side. They found these solid-state physicists in theorist Mark Stockman’s group at Georgia State University in Atlanta, USA. This collaboration was facilitated by Augustin Schiffrin, an American who had just relocated to Garching to conduct attosecond re-search himself.

In the lab in Garching, the research team used quartz to explore whether this insulator can be made conductive for a short time when bombarded with extremely intense femtosecond light flashes – without being destroyed. The researchers were also interested in how the flashes of light influence its optical properties. They tracked this momen-tary change with even shorter attosec-ond flashes.

To do this, Bothschafter and Schul-tze developed an experiment in which the experts in Garching reduced their record to a pulse duration of just 72 at-toseconds. The researchers sent these attosecond flashes to their probe, syn-chronized with femtosecond light puls-

At the source of the attopulses: Elisabeth Bothschafter calibrates the chamber in which the attosecond pulses are created.

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es. This gave them the high-precision tool necessary to scan the exact form of the femtosecond light pulses they used. This was the key to correctly in-terpreting the change these pulses caused in the quartz.

QUARTZ SURVIVES BRIEF ELECTRIC INFERNO

The physicists used the attopulses to analyze in real time and in detail what happened in the barely 200-nanome-ter-thick quartz probe when bombard-ed with light. They found that the in-tense femtosecond laser pulse, which is an electromagnetic wave, produced an extremely strong electric field in the probe. Martin Schultze compares it with the field of an overhead high-volt-age line connected between two elec-

trodes spaced just a few thousandths of a millimeter apart. Every known mate-rial would vaporize immediately.

The quartz survived solely because this electric inferno raged inside it for only a few femtoseconds. It was, quite simply, too sluggish to notice it. The or-deal was long since over and done be-fore its atoms could drift apart. For the electrons, however, it was quite a differ-ent story. Some of them followed the field of the femtosecond pulse like dogs on a leash. As a result, they temporari-ly produced an electric current in the quartz that immediately subsided again at the end of the femtosecond pulse. Then the quartz became a normal insu-lator again.

“This means that, using the light pulse, it is possible not only to turn on conductivity in the inconceivably short

span of a few femtoseconds, but also to turn it off again,” says Krausz. The lat-ter was crucial, as this doesn’t happen in semiconductors.

Stockman’s group showed that, in this situation, electrons move accord-ing to a completely different mecha-nism than in semiconductors. In the latter, they can be kicked into the con-duction band within femtoseconds, but then, from the perspective of attosec-ond physics, they stay there for an eter-nity: namely one thousand to ten thou-sand times longer than a femtosecond. In quartz, however, the electrons im-mediately follow the electric light field away from their atoms, and jump back just as quickly as soon as the field fades away. Thus, with this method, the re-searchers obtained an electric switch that acts extremely rapidly – within femtoseconds.

The physicists in Garching and their collaborative partners initially addressed these processes as basic re-searchers, and in doing so, they uncov-ered something completely new: nev-er before has anyone produced such an exotic state in an insulator. In the long term, this discovery could revolution-ize electronics, since it is the relative-ly sluggish persistence of electrons in the conduction band of semiconduc-tors that limits the switching speed of conventional transistors. In the lab,

A piece of fused silica, with a thickness of just a few nanometers and mounted in a black frame (image center), becomes conductive when struck by a red laser pulse. With a subsequent attosecond pulse, it is possible to measure how fast the fused silica becomes an insulator again. G

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Light produces electricity: Using a very intense, dark-red laser flash, Max Planck physicists make a quartz prism that they have evaporated with gold electrodes on two sides temporarily conductive. With a second, substantially weaker pulse, they push the briefly mobile electrons to an electrode. Current flows.

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GLOSSARY

Attosecond metrology: This technology, which is also referred to as attosecond chronos-copy, makes use of attosecond pulses to analyze extremely fast processes, such as the movements of electrons in atoms, molecules and crystalline solids.

Attosecond pulses: Flashes of light that last only a few billionths of a billionth of a second. In one attosecond, light covers a distance of 0.3 nanometers (one nanometer is one millionth of a millimeter). This corresponds to the diameter of a water molecule.

Femtosecond pulses: Flashes of light that last one millionth of a billionth of a second – about one thousand times longer than attopulses.

Pump-probe method: In this method, an initial laser flash, the pump laser, starts a process such as a chemical reaction. After a precisely determined delay, a second flash, the probe pulse, takes a photo of the process. The procedure is repeated with varying delay times between the pump and the probe laser pulses, creating a series of individual images that can be compiled into a film.

TO THE POINT● Using laser pulses that flare for just a few attoseconds, Ferenc Krausz and his

colleagues at the Max Planck Institute of Quantum Optics track the movements of electrons.

● The researchers have directly observed how quickly electrons pass through a single layer of atoms. Such insights contribute to the development of faster electronic switching elements.

● Extremely intense femtosecond flashes of red light briefly transform electrically insulating quartz into an electrical conductor. As an analysis with attopulses showed, the switching process takes just a few femtoseconds. With this mecha-nism, electronic components could perform operations many times faster.

using extremely small nanostructures, conventional transistors already reach speeds of hundreds of billions of cycles per second. That’s roughly one order of magnitude faster than established computer microprocessors today.

“Our discovery would make it pos-sible to achieve circuit times that are another ten thousand times faster,” says Krausz, summing up. This would allow computers to process vast amounts of data in real time. Of course, this would also require being able to min-iaturize the meter-long short-pulse la-sers that drive the switching opera-tion. “For that reason – and for a few others, as well – this is currently just an interesting prospect for the future,” he emphasizes.

For Krausz, as a Max Planck scien-tist, the focus is on the knowledge gained. With the new attosecond mea-suring technology, Reinhard Kienberg-er’s team just recorded in real time how electrons dart through individual atom-ic layers of a crystal lattice. This “jour-ney” is of key importance for physics and technology. Direct observation can be decisive in helping to one day devel-op significantly faster electronic switch-ing elements and microprocessors.

https://youtu.be/Ybk3JCunrxw

Filmmakers in the quantum world: Ferenc Krausz is the pioneer of attosecond metrology, which can be used to film the movements of electrons. The two vacuum chambers in the foreground and to Krausz’s left serve as the filming locations: these are where the experiments take place.

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